Do you mean siRNA? You can look into literature. You may find siRNA that is already designed for Stat3 in the species of your interest. Here is a review on siRNA and miRNA. Here are guidelines for siRNA design. There are some online tools that can help.
If you mean finding a microRNA which targets stat3 mRNA, I recommend miRWalk 2.0 online tool. It integrates many algorithms together so that you can choose a microRNA which is common among all database.